Turns out you can pull aggro on the mobs inside the arena, if they clip through the forcefield and kill the player, the player respawns
inside the arena. Mobs inside there are unkillable (you can knock 'em into the fire pit trap and they still won't die!), and the forcefield doesn't drop.
http://img708.imageshack.us/img708/8389/gameclient2013052115155.jpg
Hilarious.